Solutions to Chapter 2 (p. 39)

For the excercise on page 25, just type each line into ghci.

Page 39:

#1. The function can only select an element from the list: there's no other way it can obtain a value of type a; in particular, it cannot create new values of this "unknown" type.

Thus, possible valid behaviors include returning the first element, returning the second element, etc.; returning the last element, returning the last-but-one element, etc.; returning the middle element; and so on.

Interestingly, a function with this signature in Haskell cannot return a "random" element from the list, because (for functions with no IO) the same value must be returned every time the function is called with the same input.

#2. Using if:

lastButOne :: [a] -> a
lastButOne (x:xs) = if length xs == 1
                    then x
                    else lastButOne xs

With pattern matching (introduced in the following chapter), this can be simplified as:

lastButOne :: [a] -> a
lastButOne (x1:[x2]) = x1
lastButOne (x:xs) = lastButOne xs

#3. For either version, trying this on a too-short list (e.g., lastButOne "no") results in the error message "Non-exhaustive patterns in function".